Processing of planar targets by means of electrically charged particles has been investigated since the 1980s. One important application of particle-beam lithography is in semiconductor technology. Therein, lithography apparatus are used to define structures on a target, e.g., a silicon wafer. (Throughout this disclosure, the terms target and substrate are used interchangeably.) In order to define a desired pattern on a substrate wafer, the wafer is covered with a layer of a radiation sensitive photoresist. Afterwards, a desired structure is imaged onto the photoresist by means of a lithography apparatus, and the photoresist is then patterned by partial removal according to the pattern defined by the previous exposure step and then used as a mask for further structuring processes such as etching. In another important application the pattern may be generated by direct patterning without a resist, for example ion milling or reactive ion beam etching or deposition.
In a maskless multibeam apparatus, the feature shapes are defined by means of a beamlet array on a target through the amount of exposure dose imparted to each pixel. Each beamlet is switched on or off according to a desired pattern, and the beamlets are moved over the target. In the course of subsequent exposure steps, the desired layout is composed on the target. For instance, with a resist-based method, if a pixel receives an exposure dose exceeding the (given) resist-development threshold, that pixel is exposed; otherwise the pixel is not exposed. The feature shape is thus defined by the spatial distribution of exposed and non-exposed pixels.
The layout data is usually generated in polygonal structures. For the exposure of resist images by means of a maskless pattern writing tool the layout data is converted into a pixel image data (rasterization). Therefore, the technology for maskless took requires specific methods for data preparation. One feature of the maskless tool concept is that each pixel requires the same amount of time regardless of whether it is exposed or not.
The number of pixels required to obtain a sufficiently good feature resolution at standard chip sizes is quite high and remains a challenging task. Therefore, the storage of the complete rasterized image data is not feasible. Instead, the layout data are processed in an online rasterization which employ simple algorithms, which take only short runtime. These algorithms will also have to provide the capability to reduce or even eliminate the effects of possible defects on the APS, in particular so-called always-closed and/or always-open failures.
In 1997, I. L. Berry et al., in J. Vac. Sci. Technol. B, 15(6), 1997, pp. 2382-2386, presented a writing strategy based on a blanking aperture array and an ion projection system. Arai et al., in U.S. Pat. No. 5,369,282, discuss an electron beam exposure system using a so called blanking aperture array (BAA) which plays the role of a pattern definition means. The BAA carries a number of rows of apertures, and the images of the apertures are scanned over the surface of the substrate in a controlled continuous motion whose direction is perpendicular to the aperture rows. The rows are aligned with respect to each other in an interlacing manner to that the apertures form staggered lines as seen along the scanning direction. Thus, the staggered lines sweep continuous lines on the substrate surface without leaving gaps between them as they move relative to the substrate, thus covering the total area to be exposed on the substrate.
Starting from Berry's concept, E. Platzgummer et al., in U.S. Pat. No. 6,768,125, presented a multi-beam direct write concept dubbed PML2 (short for “Projection Maskless Lithography”), employing a pattern definition device comprising a number of plates stacked on top of the other, among them an aperture array means and a blanking means. These separate plates are mounted together at defined distances, for instance in a casing. The aperture array means has a plurality of apertures of identical shape defining the shape of beamlets permeating said apertures, wherein the apertures are arranged within a pattern definition field composed of a plurality of staggered lines of apertures, wherein the apertures are spaced apart within said lines by a first integer multiple of the width of an aperture and are offset between neighboring lines by a fraction of said integer multiple width. The blanking means has a plurality of blanking openings arranged in an arrangement corresponding to the apertures of the aperture array means, in particular having corresponding staggered lines of blanking openings. The teaching of the U.S. Pat. No. 6,768,125 with regard to the architecture and operation of the pattern definition device is hereby included as part of this disclosure by reference.
The PML2 multi-beam direct write concept allows for a large enhancement of the writing speed compared to single beam writers. This arises from the reduction of the required current density, the diminished importance of space charge due to the large cross section, the enhanced pixel transfer rate due to the parallel writing strategy, and the high degree of redundancy possible using a plurality of beams.
The U.S. Pat. No. 7,276,714 of the applicant/assignee discloses a pattern definition means for particle beam processing, comprising at least an aperture plate and blanking means. The apertures in the aperture plate are arranged in “interlocking grids”, wherein the apertures are arranged in groups in squares or rectangles whose basic grids are meshed together. This means that the positions of the apertures taken with respect to a direction perpendicular to a scanning direction and/or parallel to it are offset to each other by not only multiple integers of the effective width of an aperture, as taken along said direction, but also by multiple integers of an integer fraction of said effective width. In this context, “scanning direction” denotes the direction along which the image of the apertures formed by the charged-particle beam on a target surface is moved over the target surface during an exposure process.
The “interlocking grids”-solution allows a finer resolution on the target surface even though the individual spots formed by each image of an individual aperture are not decreased in size. Particular values of the fractional offsets are integer multiples of ½K times the effective width of an aperture, where K is a positive integer.
Furthermore, the U.S. Pat. No. 6,768,125 and U.S. Pat. No. 7,276,714 describe the generation of grey scales by subsequent exposures of one pixel on the target by multiple apertures located in line. Thus, a shift register approach can be effectively applied to create grey scale patterns, i.e., exposure levels interpolated between a minimal (‘black’) and maximal (‘white’) exposure dose.
The state-of-the-art PML2 concept is a strategy where the substrate is moved continuously, and the projected image of a structured beam generates 100 percent of the grey pixels by subsequent exposures of apertures located in line. To realize grey levels, the total amount of apertures in line is subdivided into columns, the number of columns corresponding to the number of desired grey levels. In a recent variant described in the DE 10 2008 015 305 A1=US 2008/0237460 A1 by the applicant/assignee, a so called “trotting mode” writing strategy is proposed in which for each pixel one or a few beams along the (mechanical) scanning direction are used to generate the entire set of the grey pixels. The advantage of this variant is the reduced complexity of the CMOS structure and improved data management.
The particle optical system is generally non-ideal, which means that the system has imaging defects, in particular image distortions and blur variations, which additionally may very over time, temperature and image position. In order to account for these imaging defects, rasterization algorithms are desired which can provide blur-independent writing possibilities and/or the capability to include an image pre-distortion, which is designed to compensate for optical distortions. Additionally the beam deflection angles shall be kept low to keep the distortion effects of the optical system also low.
However, the “trotting mode” method according to the above mentioned patent application has some very specific requirements on the APS layout, in particular with regard to the order of interlocking (i.e., over sampling), the order of the redundancy and the size of the aperture, which directly impacts on the aperture layout. A change of one of these parameters is generally not possible without altering the layout of the blanking plate.
It is an objective of the present invention to improve the “trotting mode” writing strategy so as to simplify the imaging strategy and further allow a simple mapping from polygonal structures to grey level data that are independent of the actual blur.